The present invention relates to an electrical discharge machining device comprising a tool electrode and a workpiece electrode forming the poles of a machining gap, at least one voltage/current source connected by an electrical circuit to the tool electrode and to the workpiece electrode and configured to generate electrical pulses and to establish the initiation of electrical discharges between the tool electrode and the workpiece electrode.
In particular, the tool electrode used can be a wire stretched between two guides. The main concern hereinafter will be superfine surface finishing by electrical discharge machining using a wire electrode that allows the finest surface conditions to be obtained.
In order to cut out a workpiece by electrical discharge machining using a wire, several passes are usually made; firstly, the rough-cut pass opens a passage for the wire; the surface condition obtained is very rough; in addition, the size of the workpiece obtained is purposely over-dimensioned in order to allow the subsequent passes, for fine finishing and superfine finishing, to approach the final dimensions and to produce a smoother surface state.
The majority of electrical discharge machining tools comprise two voltage/current generators; one designed to promote the initiation of the discharges; the other of higher power designed to supply the energy for the most erosive discharges. In superfine surface finishing mode, it is desired to reduce the roughness of the surfaces obtained by electrical discharge machining and hence to decrease the energy of the eroding discharges. Consequently, normally only the ‘discharge initiation’ generator is used, the relays connecting the high-power generator to the machining region remaining open.
Here, a problem is encountered associated with the current lines that connect the generator or generators to the workpiece and wire electrode. These lines are normally coaxial cables whose essential property is to have a low inductance that allows the rough-cut generator to produce current pulses with very steep edges of the order of 1000 amps per microsecond. However, this low inductance of the lines no longer provides a clear advantage during surface finishing regimes. Worse still, the coaxial cables comprise high distributed capacitances which form energy reservoirs that are incompatible with the surface finishing regimes.
It is known to those skilled in the art that the discharge initiation generator applies a voltage to the machining gap that is high enough to cause the discharge initiation without being able to deliver a high current, whereas the rough-cut generator behaves as a high current source as soon as the discharge is initiated. The discharge initiation generator applies a voltage, for example of 80 to 240 V, for an indeterminate time until the avalanche phenomenon that is often described occurs. In superfine surface finishing mode, the total energy of the discharge does not only depend on the pulse of current, as low as it is, delivered by the discharge initiation generator, but depends above all on the sum of the energies contained in the distributed capacitances connected to the terminals of the gap and to which the initiation voltage is applied, which capacitances empty their energy into the ionized channel as soon as the arc strikes.
The main problem in superfine surface finishing machining consists in localizing the stray capacitances which can discharge their energy across the gap when the arc strikes, then in blocking or attenuating this energy. The patent application EP 1 193 016 A2 illustrates some typical scenarios. Notably, in FIG. 1 of this document, for each of the stray capacitances shown, a current loop passing through the gap can be found by which the energy of the capacitance in question can be transferred into the eroding discharge when it strikes. By opening the switches disposed between the rough-cut generator and the gap, the effect of multiple stray capacitances on the machining process is blocked. The rough-cut generator with its coaxial cables is disconnected. Only a second surface finishing generator, which can be the discharge initiation generator, is connected to the gap so as to minimize the distributed stray capacitances attached to all the lines. By inserting an insulating plate between the workpiece to be machined and its holder, a capacitor is created whose capacitance will attenuate the effect of a stray capacitance of the wire electrode and also of the whole unwinding and removal system for the wire, with respect to ground. Only the capacitance that includes the capacitance of the gap itself, between the wire and the workpiece, can neither be attenuated nor blocked. The representation of the problem, such as is described in EP 1 193 016 A2, makes apparent neither the distributed stray capacitances attached to the lines between the surface finishing generator and the gap, nor those attached to the surface finishing generator, assumed to be negligible here.
Unfortunately, it turns out that these capacitances cannot be considered as insignificant.